BRD4, originally identified in association with mitotic chromosomes and implicated in cell-cycle control, has key roles as a mitotic bookmark and as a passive scaffold for transcription factors. BRD4 has an extended role: an intrinsic HAT activity distinct from those of other known acetyltransferases. BRD4 acetylates lysines on the tails of H3 and H4 and, importantly, on H3 K122, a lysine residue positioned in the globular domain of the histone octamer, where the histone-DNA interaction is strongest. BRD4 colocalizes with H3K122ac genome wide, and its acetylation of H3 leads to nucleosomal disassembly and nuclear expansion, consistently with chromatin decompaction. BRD4 preferentially decreases nucleosome occupancy around the genes that it is known to bind and regulate, such as MYC, FOS and AURKB. Furthermore, conditional deletion of BRD4 in the thymus results in decreased H3K122ac levels and perturbations in thymic development. Thus, BRD4 is an active remodeler of chromatin as well as an active transcriptional regulator. Nucleosomal disassembly and chromatin decompaction mediated by BRD4 HAT activity occur preferentially at its target loci and at sites where BRD4 binds and where H3K122ac is enriched. Although we found that BRD4 decompacted chromatin at its known targets, such as MYC, FOS and AURKB genes, it did not markedly affect nucleosome occupancy around the AURKA gene, which it does not regulate8. A previously reported 95% correlation between BRD4 occupancy and DNase I-hypersensitivity sites genome wide34 is consistent with our conclusion that BRD4 clears nucleosomes at its target gene loci. The finding that BRD4 HAT activity decompacts chromatin resolves an apparent paradox in the literature. It has been reported that a truncated isoform of BRD4 insulates chromatin from decompaction. We suggest that this isoform, which lacks the HAT catalytic domain, is a competitive inhibitor of full-length BRD4, which has HAT activity. Accordingly, overexpression of the truncated BRD4 isoform, containing only the N-terminal bromodomains, results in the aggregation of chromatin into highly compact regions. However, in a previous study, depletion of BRD4 has been found to result in increased MNase sensitivity, thus leading the authors to conclude that BRD4 is necessary for higher-order chromatin structure. We speculate that this effect was not due to BRD4-mediated compaction but instead to chromothripsis induced either by premature chromosome condensation or by an inability to decondense. That is, BRD4 overexpression and depletion both lead to altered chromatin structures, but these effects have different underlying causes. BRD4 also acts as a critical regulator of transcription, both by recruiting PTEFb to promoters and by phosphorylating S2 of the Pol II CTD through its intrinsic kinase activity, a modification necessary for transcriptional elongation. Our discovery that BRD4 has intrinsic HAT activity that regulates chromatin remodeling leads to the conclusion that BRD4 actively links chromatin remodeling and transcription. We speculate that BRD4 contributes to remodeling of chromatin first through its binding to H3K14ac at promoters or enhancers and second by acetylation of both tail lysines and H3 K122 in the globular domain of adjacent histone octamers, thus leading to nucleosome eviction around the TSSs of target genes. The decreased nucleosomal occupancy allows access to the transcriptional machinery and recruitment of transcription factors, including PTEFb, thereby resulting in transcription activation. After transcription initiation, BRD4 triggers pause release through phosphorylation of the Pol II CTD